Coverage Dependence of the Level Alignment for Methanol on TiO₂(110)

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Electronic level alignment at the interface between an adsorbed molecular layer and a semiconducting substrate determines the activity and efficiency of many photocatalytic materials. We perform G₀W₀ calculations to determine the coverage dependence of the level alignment for a prototypical photocatalytic interface: 1/2 and 1 monolayer (ML) intact and dissociated CH₃OH on rutile TiO₂(110). We find changes in the wavefunction's spatial distribution, and a consequent renormalization of the quasiparticle energy levels, as a function of CH₃OH coverage and dissociation. Our results suggest that the occupied molecular levels responsible for hole trapping are not those observed in the ultraviolet photoemission spectroscopy (UPS) spectrum. Rather, they are those of isolated CH₃O on the surface. We find the unoccupied molecular levels have either 2D character with weight above the surface at 1 ML coverage, or significant hybridization with the surface at 1/2 ML coverage. These results suggest the resonance observed in the two photon phooemission (2PP) spectrum arises from excitations to unoccupied "Wet electron" levels with 2D character.